Air source heat pumps take heat from the outdoor air and transfer the heat by direct vaporization through a heat exchanger. It works like a fridge, which uses the refrigerant to extract heat from the inside to keep food cold, the air heat pump extracts heat from the outside air and uses it to heat your home and water. Even on a cold day the air has sufficient heat to vaporize the refrigerant. These pumps require far less installation than other types of pumps and are generally cheaper also. They are especially suited for retrofitting in existing homes. An air source heat pump can reduce your heating bill by approximately 70%, with the remaining energy usually provided by another heat source (oil/gas/electricity).
When properly installed, an air-source heat pump can deliver up to four times more heat energy to a home compared to the electrical energy it consumes. This is possible because a heat pump moves heat rather than converting it from a fuel, like in combustion heating systems. The existing heating system should be surveyed by your installer as not all radiators are suitable for use with heat pumps; it is recommend that Underfloor Heating (with sufficient insulation) or low temperature radiators are used.